Roof snow accumulations in excess of specified project design loading criteria can cause significant distress to your building structural system. Snow will build up in areas around firewalls, parapet walls, valleys, dormers, and on lower roof levels where a roof step occurs. Since the density of snow varies depending on weather conditions during and after a snow fall, it is not possible to determine a single value for the allowable height of snow that a building can safely support. In addition, the underlying snow density increases due to melting from the building heat loss and as water is absorbed from the melting snow above. As weather and temperature changes continue, ice may build up under the snow layers, further increasing the building roof loading intensity. This ice build up also causes additional water back-up on the roof deck. The most severe condition occurs when rain falls on a roof system already loaded by snow. In this case, the snow absorbs the rain water, and loads can approach the weight of water (62.4 pounds per cubic foot, or 5.2 pounds per inch of depth). This condition must be monitored with extreme caution.
The following procedure may be used as a guideline for responding to roof overload conditions due to extreme snow and ice build up conditions:
Garage Buildings does not make any recommendation on when to remove snow from roofs. It is up to the individual property owner to consider the benefits and dangers of snow removal and decide their own course of action. Remember to consider the depth and relative moisture content of your snow and the capacity of your roof structure in making your decision to remove snow or not. It is also recommended to review the “Snow Removal” section of the MBMA Metal Building Systems Manual, current version. Also refer to the ICBEST – 2001 article ‘Minimizing The Adverse Effects of Snow and Ice on Roofs’.